P1304: THE NOVEL TUMOR SUPPRESSOR SAMHD1 CONTRIBUTES TO ANTI-TUMOR RESPONSES IN A CGAS-STING-DEPENDENT OR INDEPENDENT PATHWAY IN HODGKIN LYMPHOMA (HL)
Authors: I. Xagoraris, G. Kokaraki, K. Stathopoulou, P. Xyderou, A. Kurzay, N. Herold, A. Lundqvist, G. Z. Rassidakis
Journal: HemaSphere (2022)

Background: The SAM domain and HD domain 1 (SAMHD1) protein is a deoxynucleoside triphosphate (dNTP) triphosphohydrolase, which depletes the intracellular dNTP substrates. Mutations of SAMHD1 gene have been linked to Aicardi-Goutières syndrome (AGS) and have been detected in a subset of chronic lymphocytic leukemia, mantle cell lymphoma, and T-prolymphocytic leukemias. Therefore, SAMHD1 may play a role in lymphomagenesis as a tumor suppressor. We have recently reported (Xagoraris et al, Br J Haematol 2021;193(3):488) that SAMHD1 is expressed by the neoplastic Hodgkin and Reed Sternberg cells (HRS) and correlates with unfavourable clinical outcome in classical Hodgkin lymphoma (cHL). Cyclic GMP-AMP (cGAMP) synthase (cGAS) is a cytosolic DNA sensor that activates innate immune responses through production of the second messenger cGAMP, which activates the adaptor protein STING. The latter activates TBK1 and IKK kinases that, in turn, activate IRF3 and NF-κB, which induce expression of interferons and other cytokines involved in anti-tumor immune responses. SAMHD1 may reduce cytosolic DNA of either exogenous or endogenous origin, which can trigger activation of the cGAS-STING immune response pathway. Aims: This study aimed to investigate the potential role of SAMHD1 in anti-tumor immune responses in cHL. Methods: The in vitro system included 5 cHL cell lines (MDA-V, L-1236, HDLM-2, L-428 and KM-H2). Expression and activation (phosphorylation) of cGAS-STING pathway proteins at baseline and experimental conditions were analysed by quantitative RT-PCR and Western blot analysis at the RNA and protein level, respectively. The cHL cell lines were treated with VPX (or control DX) viral proteins to eliminate SAMHD1 as previously described (Herold et al, Nat Med 2017;23(2):256), and with a CDK inhibitor (Palbociclib) to restore SAMHD1 activity through dephosphorylation. Treatment with a STING agonist and TBK inhibitor were also used alone or in combination with other agents. Silencing of STING gene was performed using transient transfection of the cells with specific STING siRNA. The cGAS-STING-associated anti-tumor immune responses were evaluated by assessing the RNA levels of interferon beta (IFN-β), CXCL10, interferon gamma (IFN-γ), STING, as well as, a control gene (GAPDH) with qRT-PCR. Results: STING expression at the mRNA and protein level was substantially higher in L-1236, HDLM-2, and L-428 compared to other cHL cell lines. Depletion of SAMHD1 by VPX resulted in significantly lower levels of IFN-β and CXCL10 in cHL cells, which was linked to decreased NK cell killing of the cHL cells in vitro. Inversely, treatment with Palbociclib resulted in SAMHD1 dephosphorylation thus restoring SAMHD1 activity, which was associated with increased but differential expression of IFN-β and CXCL10 genes among the cHL cell lines. These effects were associated with activation of the cGAS-STING pathway (TBK1 phosphorylation) in cHL cells. Treatment with STING agonist alone stimulated immune responses by increased IFN-β and CXCL10 levels indicating functional cGAS-STING anti-tumor immune response pathway in cHL. Inversely, STING gene silencing resulted in decreased CXCL10 but not IFN-β gene expression. Summary/Conclusion: SAMHD1 contributes to anti-tumor immune responses in cHL, in part, through modulation of the cGAS-STING pathway. As STING agonists and Palbociclib are currently used in clinical trials for certain malignancies, our findings might have direct therapeutic implications in patients with cHL.